Known combustion catalysts are usually prepared from a monolithic substrate of ceramic or metal on which a fine layer of catalyst support material consisting of refractory metal oxides, usually aluminum oxide, and promoter oxides, usually rare earth oxides, are deposited.
Catalytic converters containing washcoated substrate are located downstream of the exhaust manifold in what is known to those skilled in the art as underfloor, close coupled or manifold mounted converters. The exhaust gas from a defect free engine contains roughly equivalent amounts of reducing species (carbon monoxide and hydrocarbons) and oxidizing species (nitrogen oxides). The reducing species are preferentially consumed by chemical reduction of the oxidizing species, resulting in exhaust emissions below established Federal and State limits. However, vehicles that have degraded fuel control, e.g., heavily aged vehicles and/or vehicles having used poor quality fuels, may emit much more reducing species than oxidizing species. Exothermic combustion of high concentrations of reducing species may lead to premature deactivation of the catalytic converter before the vehicle reaches the required 125,000 mile durability. Prevention of the excess hydrocarbon from reaching the underfloor catalytic converter is greatly preferred.
During the combustion of exhaust gas catalytic converters are often subjected to exhaust gas temperatures reaching 800° C. and higher. Exothermic combustion reactions often increase the 800° C. exhaust gas temperature to above 1,050° C. on the catalyst bed. As vehicles age and fuel control worsens, catalyst bed temperatures may exceed 1,200° C. In particular, thermal cycling of combustion catalysts above 1,100° C. degrades the low temperature catalytic activity. Possible causes for the degradation of low temperature performance include sintering of the catalyst support, catalyst erosion, and vaporization and encapsulation of the active precious metal phase(s). For example, catalyst beds that have reached more than 1,100° C. very often do not begin catalytic combustion until exhaust temperatures reach above about 450° C. Since the exhaust stream is at or below 500° C. for a significant amount of time, the amount of pollutants emitted greatly increases when the low temperature combustion activity lost.
There remains a need for improved exhaust treatment devices, systems, and methods.